At present, this type of heat exchanger has several problems relative to heat exchange efficiency. Firstly, the current multi-tube heat exchangers cannot guarantee the correct flow of the fluid around all of the tubes. Secondly, there may be thermal gradients between the various tubes which have a negative effect on the general efficiency of the heat exchange.
It should also be noticed that no type of existing tube, if used in a heat exchanger, is able to overcome said problems.